Simple OpenLayers map

OL.js

var map;

function init(){
    //defining the projection and adding a map to an element

    var sphericalMercatorProj = new OpenLayers.Projection('EPSG:900913');
    var geographicProj = new OpenLayers.Projection('EPSG:4326');
    map = new OpenLayers.Map('myMap');

    //adding layers
    //only adding the default OSM layer here
    map.addControl(new OpenLayers.Control.LayerSwitcher());
    mylayer = new OpenLayers.Layer.OSM();
    map.addLayer(mylayer);

    //adding a new layer of markers
    var markers = new OpenLayers.Layer.Markers( "Markers" );
    map.addLayer(markers);

    //Loading data
    //size (width,height) of the icon in pixels
    var size = new OpenLayers.Size(21,25);
    var offset = new OpenLayers.Pixel(-(size.w/2), -size.h);
    //the offset is used to place the base of the pin on the actual coordinates
    //if no offest defined, the center of the pin will be placed on the coordinates
    var icon = new OpenLayers.Icon('http://www.openlayers.org/dev/img/marker.png', size, offset);
    
    $.ajax({
        'async': true,
        'global': false,
        'url': "./data/data_heatmap.min.json",
        'dataType': "json",
        'success': function (data){
                            var dataLength = data.length;
                              console.log(dataLength);
                            //iterating through each point
                            while(dataLength--){
                                markers.addMarker( new OpenLayers.Marker(
                                    new OpenLayers.LonLat(data[dataLength].lon, data[dataLength].lat)
                                    .transform(new OpenLayers.Projection('EPSG:4326'), map.getProjectionObject())
                                    ,icon.clone()));
            }
        }});

    //final display settings go here
    var mapCenter = new OpenLayers.LonLat(2.0,46.0);
    map.setCenter(mapCenter.transform(
        new OpenLayers.Projection('EPSG:4326'), 
        map.getProjectionObject()),6);

};

window.onload = function() {
    init(); 
};